In a method and apparatus for measuring activity of a tracer in a subject in a medical imaging protocol, comparable features in each of a time series of image data sets of the subject are detected. A first activity value for a first region of interest containing a first of the features in a first image data set is obtained, and a size of the first feature is measured. The first activity value is then modified using the measured size of the first feature. A second activity value for a second region of interest containing a second of the features in a second image data set is obtained. The modified first activity value and the second activity value are then combined to produce a measure of activity of the tracer over the time series.